This invention relates to a radiant tube type heater for use in a heating furnace such as a heat-treatment furnace for steels or a general purpose firing furnace.
In general, fuel oil, gas or electric heaters are used in order to directly heat a heat-treatment furnace. In such a direct-heating type furnace, it is difficult to control the furnace at a constant temperature.
To better maintain a constant temperature, a radiant tube type heater is often used in a heating furnace. One type of conventional radiant tube heater has only a single tube. Another conventional radiant tube heater has in combination concentric inner and outer tubes.
The conventional radiant heater tubes are normally made of a heat resistant steel or a silicon carbide refractory as disclosed in Japanese Utility Model Laid-Open No. 58-119020. One such heat resistant steel is mainly a Ni-Cr steel. Examples of silicon carbide refractories are Si.sub.3 N.sub.4 -bonded SiC or clay-bonded SiC.
If the conventional radiant tube type heaters are used in an environment subject to a sudden change in temperature, problems due to thermal shock arise. Futher, in the case of heat resistant steels, the heater tube is subject to severe oxidation and scaling to the extent that it easily deforms. Thus, its life time is nomally only 3 to 6 months. In case of Si.sub.3 N.sub.4 or clay-bonded SiC, the radiant tube is easily oxidized to form a SiO.sub.2 phase or glass phase which is very susceptible to creep and cracking whereby its life time is normally only 6 to 12 months.